The present invention relates to a polishing apparatus and, more particularly, to an apparatus for holding an article to be polished, for example, a semiconductor wafer. With recent rapid progress in technology for fabricating high-integration semiconductor devices, circuit wiring patterns and interconnections have been becoming increasingly fine, with spaces between wiring patterns decreasing. As wiring spacing decreases to less than 0.5 microns, the depth of focus in circuit pattern formation in photolithography or the like becomes shallower. Accordingly, surfaces of semiconductor wafers on which circuit pattern images are to be formed by a stepper require a higher degree of surface flatness.
FIG. 1 shows a conventional polishing apparatus for making the surface of a semiconductor wafer flat. The apparatus includes a polishing table 4 provided with a polishing cloth 2, for example, bonded thereto to form a polishing surface. The apparatus further includes a substrate holding apparatus 6 for holding a substrate W to be polished, for example a semiconductor wafer. An abrasive liquid Q is supplied between the polishing surface and a surface of the substrate W to be polished from an abrasive liquid supply pipe 8, and the substrate W is pressed against the polishing surface under a predetermined pressure by the substrate holding apparatus 6, while the polishing surface and the substrate W are caused to slide relative to each other to thereby effect polishing.
A typical substrate holding apparatus includes, as shown in FIG. 2, a substrate holder 10 and a drive shaft 12 connected to the substrate holder 10 at the lower end thereof through a universal joint 14. The drive shaft 12 is adapted to transmit a rotational torque and a pressing force to the substrate holder 10 through the universal joint. The substrate holder 10 is also allowed to pivot about the universal joint 14. The substrate holder 10 includes a substrate holding member 16 and a cover member 18 placed on and fastened to the substrate holding member 16 with a clearance or gap S being formed therebetween. The substrate holding member 16 is adapted to hold a substrate W against its lower surface under the influence of a vacuum generated in the substrate holder 10 and against the substrate. Over the substrate holding member 16 and the cover member 18 there is provided an annular pressure member 20 to impose a downward pressure thereon. The substrate holding member 16 is provided along its outer periphery with a guide ring 22 to cooperate with the substrate holding member to define a recess for receiving the wafer W.
The upper surface of the substrate holding member 16 has a center recess 24 and an annular step surface 26 surrounding the recess 24. The cover member 18 has on its lower surface a centrally projecting portion 28 for engagement with the center recess 24 of the substrate holding member 16 and has along its outer periphery an annular flange 30 to be fastened to the step surface 26 of the substrate holding member 16 using bolts. The cover member 18 also has on its upper surface an annular projection 34 positioned in coaxial relation therewith, and a recess 32 surrounded by the annular projection 34. A rubber sheet 37 is provided between a step surface 36 of the cover member 18 and the flange 30 to seal the gap S.
The gap S is fluidly connected to a fluid connection path 40 formed in the pressure member 20, a fluid connection structure (not shown), and to the lower surface of the substrate holding member 16 through a plurality of holes 38 vertically extending through the substrate holding member 16. The fluid connection path 40 is adapted to be selectively connected to a vacuum source and a pressurized fluid source (not shown) so that a vacuum and pressurized fluid are selectively applied to the lower surface of the substrate holding member 16.
The drive shaft 12 has a drive member 68 with a flange portion 66 extending radially outwardly therefrom. The universal joint 14 is provided between the drive member 68 and the cover member 18, and about which the substrate holder 10 tilts while a force is transmitted to the substrate holder 10. The universal joint 14 includes a spherical bearing 70 and a rotation transmission device 72 for transmitting a rotational drive force to the substrate holder 10 from the drive shaft 12.
The spherical bearing 70 includes a ball bearing 78 and hemispherical recesses 80, 82 formed respectively at the center of a projected portion 76 formed on the lower surface of the drive member 68 and at the center of the recess 32 formed in the upper surface of the cover member 18, with the hemispherical recesses slidably receiving the ball bearing 78. The lowest point of the ball bearing 78 received in the hemispherical recess 82 is positioned below the step surface 26 of the cover member 18. Due to the provision of the recess 24 in the substrate holding member 16 and the projected portion 28 of the cover member 18, the thickness of the substrate holder 10 is able to be made smaller. Further, the positioning of the universal joint 14 in the recess 32 of the cover member 18 makes it possible to position the spherical bearing 70 close to the turntable 4.
However, it is desired to provide the spherical bearing 70 at a position much more closer to the turntable 4 in order to decrease the amount of rotational moment around the spherical bearing 70 generated by a friction force acting between the substrate W and the turntable 4 and imposed on the substrate holder 10 during a polishing operation, to enable a stable polishing operation.
Since a magnitude of the rotational moment is proportional to a distance between the spherical bearing 70 and the substrate W, it may be possible to decrease the magnitude by increasing the depth of the recess 32. However, this leads to a decrease in rigidity of the substrate holding member 16 with the result that the substrate holding member 16 becomes susceptible to deformation under the influence of a pressing force transmitted through the spherical bearing 70 and, as a result, it becomes difficult for a substrate to be appropriately flattened.
In particular, with an increase in size of wafers there is a concomitant increase in the amount of heat generated during a polishing operation, with the heat being transferred from the wafer to the wafer holder. Due to the large-size of the substrate holder, the mass thereof is greatly increased, however, a surface thereof from which heat is discharged is not greatly increased. As a result, the heat accumulated in the substrate holder has an adverse influence on the polishing operation.
The amount of heat generated at the center area of a substrate W is greater than that generated at the periphery of the same, and consequently a temperature gradient exists with respect to the substrate along its radial direction with the result that the heat transmitted to the substrate holding member 16 increases with a corresponding gradient. It is therefore necessary for the substrate holding member 16 to be resistant to and capable of readily discharging heat. One of the easiest ways of meeting this requirement is to decrease the mass of the substrate holding member 16.
Further, it is necessary for the substrate holding member 16 to be periodically disassembled from the substrate holder 10 in order to renew a substrate holding pad provided on the lower surface of the substrate holding member and/or to clean the substrate holding member. It is desirable from an operator""s point of view to provide a substrate holding member which is not too heavy. As substrates have have become larger in recent years, the weight of holders has become a significant problem.
This invention is made in the light of the above-stated circumstances.
An object of this invention is to provide a substrate holding apparatus comprising a substrate holder having a center axis for rotation and adapted to hold a substrate to be polished and urge the substrate against a polishing pad, a drive shaft for drivingly rotating the substrate holder about the center axis, and a joint for connecting the substrate holder to the drive shaft in such a manner that the substrate holder can tilt relative to the drive shaft, the substrate holder includes a substrate holding member and a cover member placed on and secured to the substrate holding member. The substrate holding member a lower surface for holding the substrate and an upper surface having a curved surface recess with an opening at the upper surface and a center bottom portion which is the deepest in the recess. The cover member has a lower surface with a projected portion inserted into the curved surface recess, and an upper surface having a recess extending towards the projected portion and having a bottom surface through which the axis of rotation of the substrate holder vertically extends. The joint is provided at the bottom of the recess of the cover member.
The recess of the cover member may extend into the projected portion so that the bottom portion thereof is positioned below the level of the opening formed in the upper surface of the substrate holding member.
The curved surface recess may have a vertical cross section configuration represented by a curve of the second order such as an oval, a circle and so on.
It is preferable that the difference between a moment of inertia relative to a reference axis passing through the joint and a moment of inertia relative to an axis parallel to the reference axis and passing through the center of gravity of the substrate holder is in the range of xc2x15% of the latter moment of inertia.
The projected portion of the cover member may be formed in a complementary fashion relative to the curved surface recess.
Another object of this invention is to provide a substrate holding apparatus comprising a substrate holder having a center axis for rotation and adapted to hold a substrate to be polished and urge the substrate against a polishing pad, a drive shaft for drivingly rotating the substrate holder about the center axis, and a joint for connecting the substrate holder to the drive shaft in such a manner that the substrate holder can tilt relative to the drive shaft. The substrate holder includes a substrate holding member having a lower surface for holding the substrate and an upper surface. The cover member has a lower surface opposing the upper surface of the substrate holding member and an upper surface connected to the joint. The substrate holding member and the cover member are fastened to each other via a first set of bolts which are arranged at a predetermined interval along a first circle coaxial with the axis of rotation, and a second set of bolts which are arranged at a predetermined interval along a second circle coaxial with and radially outside of the first circle.
The bolts of at least one of the first and second sets may be inclined from a vertical orientation.
The bolts of one the first and second sets may be tightened with a torque to provide an appropriate mechanical connection strength between the substrate holding member and the cover member, and the bolts of the other set may be tightened so as to adjust flatness of the lower surface of the substrate holding member.
It is preferable that the upper surface of the substrate holding member is provided with a curved surface recess having an opening formed in the upper surface, the lower surface of the cover member is provided with a projected portion the configuration of which is complementary to the curved surface recess of the substrate holding member and, the bolts inclined from the vertical orientation are arranged along a circle, the diameter of which is smaller than that of the opening so that the bolts extend substantially normal to opposing surfaces of the curved surface recess and the projected portion of the cover member.
A further object of this invention is to provide a substrate holding apparatus comprising a substrate holder having a center axis for rotation and adapted to hold a substrate to be polished and urge the substrate against a polishing pad, a drive shaft for drivingly rotating the substrate holder about the center axis, and a joint for connecting the substrate holder to the drive shaft in such a manner that the substrate holder can tilt relative to the drive shaft. The substrate holder includes a substrate holding member having a lower surface for holding the substrate and an upper surface. The cover member has a lower surface opposing the upper surface of the substrate holding member and an upper surface connected to the joint. The substrate holding member and the cover member are fastened to each other via a plurality of bolts which are arranged at a predetermined interval along a circle coaxial with the axis of rotation and are inclined from a vertical orientation.
It is preferable that the upper surface of the substrate holding member is provided with a curved surface recess having an opening formed in the upper surface, the lower surface of the cover member is provided with a projected portion the configuration of which is complementary to the curved surface recess of the substrate holding member, the bolts inclined from the vertical orientation are arranged along a circle, the diameter of which is smaller than that of the opening so that the bolts extend substantially normal to opposing surfaces of the curved surface recess and the projected portion of the cover member, and the projected portion of the cover member is provided on its surface with cylindrical projections provided in such a manner that each cylindrical projection surrounds the corresponding one of the bolts inclined from the vertical orientation and abuts the surface of the curved surface recess of the substrate holding member so as to form a clearance between the surface of the projected portion of the cover member and the curved surface of the curved surface recess.
The above and the other objects, features and advantages of this invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.